The conversion of a ligno-cellulosic biomass feedstock to bio-based chemical compounds which may be used to replace of products derived from petroleum, such as fuel and chemical raw materials, has attracted much attention over the last decades. Among conversion processes, biological conversion of sugars from ligno-cellulosic biomass feedstocks by means of enzymes and microorganisms is considered promising for producing fuels and chemicals at low cost with a very limited environmental impact.
A major difficulty in the conversion of a naturally occurring ligno-cellulosic biomass feedstock arises from the low accessibility of complex sugars, which are trapped inside the lignocellulose, a combination of lignin, hemicellulose and cellulose that strengthens biomass plant cells. Cellulose and hemicelluloses comprise respectively insoluble complex sugars, respectively C6-based glucans and C5-based xylans. A pre-treatment of the ligno-cellulosic biomass feedstock is therefore necessary to effectively disrupt the structure of the feedstock. Extensive research efforts have been devoted to various chemical pretreatments of biomass to overcome the barriers and to enhance enzyme accessibility to cellulose, by removing chemical components of biomass (lignin and/or hemicellulose).
Typically, mechanical, physical, biological and chemical pretreatments have been used.
Mechanical pretreatments are used to reduce the size of biomass feedstock and decrease the crystallinity of cellulose. Several well developed technologies are available for biomass size reduction, such as hammer milling, knife milling, shredding, and disk or attrition milling. The size reduction of biomass feedstock consumes a significant amount of electrical-mechanical energy, which is particularly critical in the case of hardwood and softwood ligno-cellulosic biomass. Typical energy consumption to produce chips with size of few centimeters is of the order of 50 Wh/kg. Depending on the process, the feedstock and the degree of milling, up to 500-700 Wh/Kg or more are needed to obtain chips of few millimeters. For most biomass resources, good sugars conversion efficiency has been achieved when pretreatment was applied to significantly size reduced biomass feedstock at the expense of consuming significant electric-mechanical energy. Therefore, even if size reduction is effective in increasing the accessibility of the sugars in the ligno-cellulosic biomass, the cost of electro-mechanical energy used in the process may be economically incompatible with the target price of the final product.
Chemical pretreatments are processes which have been adapted and tailored mainly from pulp and paper industries, wherein different chemicals are used to remove or modify key chemical components of the feedstock, thereby increasing the sugars accessibility. As the chemical agents, such as inorganic acids, bases, are extremely aggressive and environmentally dangerous, their use poses environmental concerns and add costs to waste management.
Steam explosion is another known pre-treatment technique in which the ligno-cellulosic biomass feedstock is first subjected to hydrothermal treatment at high temperature, followed by rapid release of the steam pressure to produce an explosive decompression to open up the biomass fibres. As the steam pressure is determined by the temperature, the ligno-cellulosic biomass is heated to temperature higher than 200° C., which degrade a portion of the sugars to degradation products, thereby limiting the conversion yield. Chemicals, such as acids, ammonia, sulfites are in some cases added to improve the effectiveness of the process.
A review of the pre-treatment processes may be found in Mosier et al., Bioresource Tech., 96(6):673-686, 2005.
It is known that feedstock comminution to chips before chemical pre-treatments strongly enhances the chemical impregnation of the feedstock and increases accessibility to sugars.
The use of chemical pretreatments followed by the application of mechanical forces is also disclosed. Many prior art documents disclose generic particle size reduction by means of mechanical forces after a chemical pretreatment, but they do not recognize the electro-mechanical energy consumption as the key factor limiting the real use of the combined chemical and mechanical processes. Thereby, the mechanism of fibrillation without wasting a significant portion of the electro-mechanical energy in heating the ligno-cellulosic biomass through useless friction forces has not been disclosed. Moreover, the improvement in sugars accessibility and hydrolysis yield are the only properties which are considered important to define or optimize the mechanical process. As a results, the disclosed mechanical processes are defined and conducted without taking into account important applicative properties, such as the capability of forming a low viscosity slurry.
WO2008131229A1 discloses a method of processing ligno-cellulosic material, comprising initial steam pretreatment to give pretreated ligno-cellulosic material with an average particle size, followed by refining to give refined ligno-cellulosic material with an average particle size, wherein the average particle of the pretreated ligno-cellulosic material is greater than the average particle size of the refined ligno-cellulosic material. The application discloses generic particle size reduction, without clarifying any fiberization mechanism and degree of particle size reduction.
WO2011044292 discloses a process for the thermal-mechanical pretreatment of biomass. The process includes subjecting a biomass feedstock including fibres containing cellulose, hemicellulose and lignin, to steam explosion steps and then subjecting the steam exploded biomass to axial shear forces to mechanically reduce the size of the fibres of the biomass to obtain treated biomass. The disclosed process uses a compounder comprising a shear zone, which allows the compounder to initially function as a mechanical polisher by imposing shear along the longitudinal axis of the biomass fibres in specially designed compounder screw elements. As an effect of the mechanical polishing (or fibre size reduction), enzymatic hydrolysis conversion is improved. The energy needed in the mechanical action on the fibres is quite high, as the best value presented in examples is 1.03 kWh/Kg of ligno-cellulosic biomass and a relevant amount is lost in heating by friction. Namely, a relevant amount of mechanical energy is lost in heating the biomass as it is necessary to remove the steam energy from the thermal reaction and mechanical steps. The described process also performs an important function of removing reaction degradation products, such as furfural, acetic acid, and other hydrophobic biomass extracts which are harmful or inhibitory to fermentation organisms. Thereby, the disclosed process degrades a certain amount of sugars to sugars degradation products which are then removed by flashing.
US2009298149A1 discloses a method using sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL). More specifically, it relates to a sulfite-based chemical process for pretreating biomass in solutions to reduce access barriers of enzymes to the lignocellulose, resulting in efficient conversion through enzymatic saccharification. Biomass chips or chops are directly subjected to sulfite pretreatment according to various embodiments, followed by a mechanical size reduction step (e.g., milling, abrading, grinding, crushing, chopping, chipping or the like). The application discloses a chemical pretreatment and mechanical size-reduction techniques to significantly reduce energy consumption in biomass size-reduction. Specifically, the inventors define as a target an electro-mechanical specific energy of 90 Wh/Kg to obtain a 90% of bioconversion efficiency. The solution proposed is a very aggressive chemical treatment to alter the biomass chemical and physical structure before conducting size reduction to fibres, fiber bundles, or powders. As stated in the application, the term “size-reduction process” is referred to the reduction of the wood chip to fibres and/or fibre bundles with lengths of about 2 mm.
In Chen et al., Biotechnology for Biofuels, 2012, 5:60, for increasing cellulose digestability during enzymatic hydrolysis and overall ethanol yield, a combination of low-severity sulfuric dilute acid pretreatment, deacetylation step and mechanical refining is studied. Deacetylation step in realized by NaOH impregnation in an additional deacetylation unit and mechanical refining is performed by means of a PFI mill. Mechanical refining was demonstrated to improve digestability of deacetylated and lower-severity-pretreated corn stover. The PFI mill is a laboratory-type mill, and it is essentially a compression unit, which, given the same energy input, produces refining effects differing significantly from a conventional disk refiner. Exerting force as compression rather than shear, results in higher internal fibrillation and lower external fibrillation and fibre shortening. No indication is given of the energy used in the mechanical refining, which is primarily expended on fibrillating the internal part of the single fibres.
In I. C. Hoeger et al., Cellulose (2013), 20, 807-818, two different pre-treated ligno-cellulosic biomass were subjected to mechanical fibrillation by stone grinding. Fibrillation refers to break out of fibre cell walls to produce macro or nanofibrills. Among other techniques, water retention value was used to measure the degree of fibrillation, as it is a measure of fibre swelling capacity and quantifies the amount of water in fibre pores and between fibres after elimination of free water. Thereby, it indicates the maximum amount of water which is retained by the ligno-cellulosic biomass. The water retention value increases in both the samples of the paper, indicating that the effect of the mechanical process is a fibre fibrillation.
In M. Wiman et al, Biotechnology and Bioengineering (2011), Vol. 108, n. 5, p. 1031, a comprehensive rheological characterization of dilute acid pretreated spruce followed by fibre size reduction by means of a knife mill is presented, accounting for the effects of water insoluble solids concentration, particle size distribution and the degree of enzymatic hydrolysis. The authors found that the rheological effects of particle size distribution could be attributed to the size of the fibres alone. The milling of the pretreated of the pretreated material resulted in significantly higher viscosity.
In S-H. Lee et al., Bioresource Technology 101, (2012), pag. 9645-9649, woody biomasses were pretreated with hot-compressed water and then micro/nanofibrillated by a twin-screw extruder to improve monosaccharide production yield. The authors found that partial removal of hemicellulose and lignin by the hydrothermal treatment effectively improved the fibrillation by extrusion. The extruded biomass had a fine fibrous morphology on a sub-micro/nanoscopic scale. The authors investigated electric power consumption, asserting that this is linearly decreasing on a logarithmic scale as extrusion discharge capacity increases, showing that the bigger the extruder the less the energy consumption, but they did not report the energy required to reduce the thermally pretreated woody biomass to a micro/nanofibrillated material.